Because of the high complexity of modern integrated circuits, and the delicacy of the processes by which they are formed, they are traditionally inspected at many different times during fabrication. As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices.
As used in the art, the term “inspection” is typically limited to an image-type inspection of the integrated circuits, rather than an electrical “inspection,” which is typically referred to as “testing.” Inspection is also performed on other types of items that are used in the integrated circuit fabrication process, such as masks and reticles. As used herein, the term “substrate” applies without limitation to integrated circuits, the wafers on which they are formed, masks, and reticles.
One powerful tool that is used in the inspection of substrates is the cold field-emission electron microscope. “Cold field” refers to the fact that the filament in the instrument's electron gun operates at room temperature. Cold field emitters have advantages in high resolution electron microscopes because they have both a relatively small virtual source size of less than about five nanometers, and a relatively low energy spread of less than about three-tenths of an electron volt. One disadvantage of cold field emitters is ion bombardment and the flicker noise that is associated with atoms moving on the surface of the tip. For example, the flicker noise from the total emission current from a HfC emitter is typically about five percent during the stable emission time (after flashing), and the probe current noise is typically larger.
The present technique for reducing the effect of flicker noise in the image is to measure the beam current in an annular region around the electron beam, and then adjust the amplitude of the detected signal as the fluctuations occur. Unfortunately, this method assumes that there is a good correlation between the measured current in the annular region and the actual probe current, which is not always a valid assumption.
What is needed, therefore, is a system that overcomes problems such as those described above, at least in part.